1. Field of the Invention
The present invention relates to a method of preparing a void type plastic pigment and, more particularly, to a method of preparing a plastic pigment usable in place of inorganic pigments, e.g., titanium oxide (TiO.sub.2) or organic pigments, e.g., a styrene plastic pigment in paints or paper coatings.
2. Discussion of Related Art
There has been widely used titanium oxide (TiO.sub.2) as a material for rendering the hiding power to paints or paper coatings.
Titanium oxide has the refractive index of 2.3, which is high relative to most other polymers whose refractive indexes are around 1.5, so that it induces a scattering of light on the paints or paper coatings, thereby providing an opacity that increases the hiding power.
Titanium oxide is generally prepared by sulfate process or chloride process, both of which are detrimental as they involve production of contaminant materials that cause environmental problems.
Titanium oxide is such an inorganic material that adds the weight of a coated product and also very expensive. As a white pigment, it is thus now increasingly replaced with organic polymer.
There are two types of synthetic pigments as organic plastic pigments which can be substituted for titanium oxide; one of them consists of particles having multi-cellular structure including titanium oxide and air encapsulated with polymeric matrices, and the other type has an enclosed void structure using a solvent of no compatibility.
However, these pigments hardly contain uniform voids, requiring extremely complex processes for their preparation and high production costs, and they are ineffective in enhancing the hiding power of a coating.
With the view to solving the problem with such pigments, there has been suggested a void type plastic pigment. This type of plastic pigment that has internal voids is provided with hard shells and thus, when it is blended into paper coating color or water paint to form a coating lamination or coating, the internal voids can be maintained without formation of films.
Water is contained in such a void type plastic pigment in liquid state and vaporized while the coating lamination or coating is dried, with consequence of internal pore formation.
Hence, the incident light can be effectively scattered by the difference of refractive index between the internal pores and their encapsulated polymeric shells to increase the hiding power and whiteness.
Compared with plastic pigments having no pores, the void-containing plastic pigments can provide a scattering of light more effectively, thus leading to a great increase in hiding power and whiteness per unit weight.
A method of preparing a latex type plastic pigment with a void structure is disclosed in U.S. Pat. No. 4,427,836 and No. 4,594,363, and Korean Patent No. 80,123.
In the U.S. Pat. No. 4,427,836, a preparation involves in the use of tertiary amines in neutralizing polymeric acid contained in the core in order to form a void structure. However, tertiary amines are offensive-smelling and detrimental to human bodies.
To overcome the problem, the U.S. Pat. No. 4,594,363 discloses another method in which amines are substituted by inorganic bases such as potassium hydroxide. This method may decrease the toxicity and environmental problem caused by using amines but involve a gelation to produce fine coagulum in the process of polymerization.
A multi-step polymerization is disclosed in the Korean Patent No. 80,123. This is a multi-step emulsion polymerization that the formation of fine coagulum or secondary cores in the course of polymerization can be inhibited in the respective steps.
To describe such a method in more detail, the multi-step emulsion polymerization involves sequential seed-polymerizing, core-polymerizing, sheath-polymerizing, and shell polymerizing after swelling of the sheath polymer. This emulsion polymerization includes the steps of: seed-polymerizing an emulsion of acryl-based monomer mixture containing monomers with acid groups of about 16% to obtain a seed latex; adding an emulsion of acryl-based monomer mixture containing monomers with acid groups of about 35% to the seed latex for 3 hours and core-polymerizing the resulting mixture to obtain a core latex; adding an emulsion of acryl-based monomer mixture containing monomers with acid groups of about 5% to the core latex for 2 hours and sheath polymerization the resulting mixture to obtain a sheath latex; swelling the latex particles by addition of an aqueous potassium hydroxide solution up to pH 7; and adding a styrene monomer emulsion to the swollen latex and shell-polymerizing the resulting mixture.
However, the void type plastic pigment prepared by this multi-step emulsion polymerization hardly has an improvement in hiding power and whiteness to a large degree.